The present invention relates to the field of magnetic recording and relates to electromagnetic transducers usually designated by the name "heads". Such transducers cooperate with a mobile magnetizable support medium in which the local variations of the magnetic condition represent the recording properly speaking and function to convert these variations into a usable electrical signal available at the terminals thereof. Symmetrically, the application of an electric signal to the terminals thereof determines a variable magnetization in the mobile magnetizable support medium which is caused to travel past these transducers.
The transfer of the reciprocal magnetism variations between such a head and a support medium is caused by this latter traveling past in front of an air gap arranged in a magnetic circuit having a generally toric shape, and the magnetic flux variations of which the magnetic circuit is the seat are either created, in the case of recording, or transformed into electric signals, in the case of reading, by means of coil-shaped windings through which passes the magnetic circuit.
In practical applications, use is often made of different heads for recording and reading, the width of the air gap and the material from which the magnetic circuit is made not being the same if it is desired to optimize the result of each operation. This is the case for example in the electro-acoustical field.
However, in other fields, such as that of magnetic data cards, for example credit cards, the space taken up by separate heads does not allow the standards defined by international regulations to be satified and it has been proposed combining the two recording and reading heads into a single case, the two air gaps forming part of magnetic circuits which are either juxtaposed or even interleaved in one another, which arrangements allow successive air gaps to be obtained very close to one another.
The use of two distinct air gaps is justified, on the one hand, by a characteristic of the magnetic recording, when it is used in the field of data cards. These latter do not lend themselves well, because of their shape, and contrary to the case of magnetic tapes, to obtaining a perfectly defined geometric tape travel. In practice, this characteristic results in fluctuations in the transverse geometric position of the support medium during travel thereof, leading to instabilities of the signal when a track magnetized by a head used for recording is caused to pass in front of the same head used for reading.
To get over these problems of mechanical tolerances of the positions of the magnetic tracks, it has been discovered that it was necessary to record a track practically three times wider than the width of the reading track, which arrangement allows a constant reading level to be obtained even if transverse fluctuations take place during the traveling of the card in front of the head.
Putting this arrangement into effect does not present special difficulties: the recording air gap is simply chosen so as to have, transversely, a length three times the length of the reading air gap.
However, as will be explained further on in detail, it has been discovered in practice that separation of the functions of the two air gaps is not completely obtained in these combined heads, although the electric recording and reading windings are separate; in fact, the two successive air gaps both record and read simultaneously, in a harmful way, the magnetic data, giving rise to risks of error, in particular in binary-type digital signals.
In practical application, it is then often necessary to give up the heads with two air gaps and to choose the simplest solution of the head with a single air gap, assuming alternately the recording and reading functions. This solution presents the disadvantage, a secondary disadvantage in the case of digital data recordings, of leading to renouncing the optimization of the performances obtained with heads having two separate air gaps; but it prohibits reading over a width narrower than that of the recorded track, a drawback the importance of which was shown earlier on.